Rotary disk filters of the afore-described type are, of course, known in the art and have the advantage that they are relatively compact and provide in a comparatively small space a large filter area which can enable withdrawal of filtrate from the individual filter cells in the suspension, to deposit solids on the filter surface (e.g. filter cloth) and to draw adherent filtrate from the solids when the latter are swung into a drying zone out of the trough.
Rinsing can be effected along the portion of the disk out of the suspension and, indeed, the solids can be treated on the filter disks with other liquids as well (see the commonly assigned, copending application Ser. No. 870,614 filed Jan. 19, 1978).
A filter which functions in this manner is also described in the brochure Scheibenfilter SSF published by Krauss-Maffei A.G. In this filter system, each sectoral filter cell is connected by a separate filtrate tube or suction tube to the control disk at which the respective tubes are connected to the respective liquid displacement or aspiration ducts by separate tubes or conduits extending through the interior of the shaft which is hollow.
The control head or disk, with which these conduits communicate, is a distribution valve subdivided into a washing zone, a drying suction zone and a cake-discharge zone at which, respectively, filtrate is pumped from the sector whose conduit registers with the first of these zones, residual fluid is aspirated by a suction pump in the second of these zones to draw air through the cake for drying purposes, and fluid under pressure is fed through the conduit aligned with the third of these zones to dislodge the filter cake from the filter material (cloth) of the disk.
With the prior-art arrangement just described, the filtrate can readily be pumped out of the sectors of the lower quadrants of the disk while air can be drawn through a sector in an upper quadrant and compressed air can be blown into a sector of an upper quadrant for the dislodgment of the filter cake.
A disadvantage of this arrangement is, however, that the provision of a large number of filter disks and a relatively large number of sectors within each disk requires an extremely large number of tubes or conduits which must pass through the shaft. The relatively dense tube bundle makes repair or replacement of the components and conduits difficult and expensive.
The problem is especially pronounced when the tubes or conduits are subjected to wear as is the case when abrasive solids are to be filtered from the suspension and/or solids are to be filtered from corrosive media.
In practice it has been found that the tubes connected to each sector last only for a brief period, e.g. several months, whereupon they must be replaced at considerable cost and by complex maintenance and repair procedures. Naturally during the replacement period, the apparatus is out of service.
Investigations have already been made to attempt to reduce the cost of such apparatus and the maintenance and repair thereof by leading the filtrate from the individual cells of adjacent or successive disks in common through the filter shaft. While this is not usually a problem for the extraction of the filtrate from the sectors immersed in the trough, it does pose a problem when compressed air is to be fed to the sector after drying for dislodging the filter cake. In practice it has been found that the compressed air of systems of the latter type is not distributed uniformly to the filter disks.
The compressed air takes the path of least resistance and is thus concentrated at the disks proximal to the control head and frequently the pressure pulse at more distal disks is insufficient to dislodge the filter cake.
When the filter is not fully dislodged from some of the disks, it is rotated again into the suspension so that additional solids build up to form a thicker filter cake in the regions more remote from the control head.
Eventually these thicker portions of the filter cake retain more moisture, are subject to less drying and, in general, interfere with the uniform operation of the system. The eventual removal of this moist filter cake can even create obstructions in the ducts which carry the filter cake away from the filter.